Bimodal Cochlear Implant Listeners’ Ability to Perceive Minimal Audible Angle Differences

Abstract

Background: Bilateral inputs should ideally improve sound localization and speech understanding in noise. However, for many bimodal listeners [i.e., individuals using a cochlear implant (CI) with a contralateral hearing aid (HA)], such bilateral benefits are at best, inconsistent. The degree to which clinically-available HA and CI devices can function together to preserve interaural time and level differences (ITDs and ILDs, respectively) enough to support the localization of sound sources is a question with important ramifications for speech understanding in complex acoustic environments. Purpose: To determine if bimodal listeners are sensitive to changes in spatial location in a minimum audible angle (MAA) task. Research Design: Repeated-measures design. Study Sample: Seven adult bimodal CI users (28-62 years). All listeners reported regular use of digital HA technology in the non-implanted ear. Data Collection and Analysis: Seven bimodal listeners were asked to balance the loudness of pre-recorded single syllable utterances. The loudness-balanced stimuli were then presented via direct audio inputs of the two devices with an ITD applied. The task of the listener was to determine the perceived difference in processing delay (the inter-device delay, IDD) between the CI and HA devices. Finally, virtual free field MAA performance was measured for different spatial locations both with and without inclusion of the IDD correction, which was added with the intent to perceptually synchronize the devices. Results: During the loudness balancing task, all listeners required increased acoustic input to the HA relative to the CI most comfortable level to achieve equal interaural loudness. During the ITD task, three listeners could perceive changes in intracranial position by distinguishing sounds coming from the left or from the right hemifield; when the CI was delayed by 0.73, 0.67, or 1.7 ms, the signal lateralized from one side to the other. When MAA localization performance was assessed, only three of the seven listeners consistently achieved above-chance performance, even when an IDD correction was included. It is not clear whether the listeners who were able to consistently complete the MAA task did so via binaural comparison or by extracting monaural loudness cues. Four listeners could not perform the MAA task, even though they could have used a monaural loudness cue strategy. Conclusions: These data suggest that sound localization is extremely difficult for most bimodal listeners. This difficulty does not seem to be caused by large loudness imbalances and IDDs. Sound localization is best when performed via a binaural comparison, where frequency-matched inputs convey ITD and ILD information. Although low-frequency acoustic amplification with a HA when combined with a CI may produce an overlapping region of frequency-matched inputs and thus provide an opportunity for binaural comparisons for some bimodal listeners, our study showed that this may not be beneficial or useful for spatial location discriminati...

Auditory Spatial Discrimination and Sound Localization in Single-Sided Deaf Participants Provided with a Cochlear Implant

“…Beijen et al [2010] tested children with a CI on one side and a hearing aid on the other side with a left-right discrimination task and showed that the children performed significantly better than chance. Zaleski-King et al [2019] tested MAA via headphones in adult participants with a CI on one side and a hearing aid on the other side and found similar results. Bilaterally provided CI participants benefit from the second CI: Senn et al [2005] showed that the participants performed better in the MAA task with two CI than with one CI.…”

“…In contrast, hearing-impaired children tested without their hearing aids did not consistently benefit from frontal positions of the sound sources in the MAA task [Meuret et al, 2017]. Zaleski-King et al [2019] measured MAA via headphones in bimodally fitted adults and showed no increasing MAA with increasing laterality. Most of the participants of the present study also gained no benefit from frontal signal presentation.…”

Impact of processing-latency induced interaural delay and level discrepancy on sensitivity to interaural level differences in cochlear implant users

“…In both studies, the temporal matching of both sides resulted in improved localization accuracy. Another study found no benefit in sound localization by matching the CI to the hearing aid delay [ 2 ]. In our study no significant correlation was found between JND-ILD and sound localization errors.…”

Cerebral Representation of Sound Localization Using Functional Near-Infrared Spectroscopy

“…The speakers spanned an angle of −90° left to +90° right with a spacing of 30°. Since more sound source directions could increase the test duration and thus cause the subject fatigue, only five of seven loudspeakers (all speakers were real and available) were used for sound presentation in this experiment ( Godar and Litovsky, 2010 ; Zaleski-King et al, 2019 ). A schematic diagram of the loudspeaker arrangement is shown in Figure 1E .…”